This invention relates to controls for direct current motor drives, and in particular it relates to motor drives where regenerative control is required.
Direct current motor drives having controls which provide regenerative braking have many applications, for example, in hoists, cranes and vehicle drives. The present invention will be described with reference to improved control for vehicle drives. Control systems are well known for controlling direct current or DC motor drives for vehicles. Such control systems provide for speed control during motoring and also provide control during regenerative braking where the motor acts as a generator and returns electrical power to the power source. One type of regenerative braking control reverses the motor field during braking, as opposed to reversing armature current, and this is termed a field reversing control. The present invention is concerned with a field reversing type control. In this type of control it has been found to be necessary, or at least highly desirable, to coordinate the control of armature current with that of field current during reversal of the field. This coordination of control avoids or reduces sparking which might occur when the field was weak but the armature current was not weak, as well as reducing the possibility of some forms of instability. There are several means of coordination of the control of field current and armature current known in the prior art. One such means controls the armature current with field reversal for regenerative braking, by reducing armature current to zero as the field current is reduced to zero in the process of reversal. Then the armature current is allowed to increase as the reversed field current increasess, at least during the transient situation where the field is relatively weak. As the field response is much slower than the armature current response, the time taken for reversal is increased in accordance with the field current response. In addition, this prior art system restricts control response during at least the initial part of field reversal.
During the time period when the field current is low and armature current is low, the motor torque is reduced quite considerably. A very low motor torque may result in instability.